During the past decade, new immunosuppressive modalities have provided important short-term allograft survival rates. Despite the increased selectivity of these agents, their overall effect is long-term suppression of the recipient's general immune responsiveness. Even with life-long administration of such suppression, a relentless annual attrition of 3-5% is observed in recipients of all types of allografts, primarily due to chronic rejection, but also because of patient non- compliance and complications of the immunosuppressive therapy itself. Moreover, not even short term survival has been achieved in xenograft recipients, since the amount of conventional immunosuppressive therapy required has led to unacceptable susceptibility to infection. Induction of donor specific tolerance following a limited period of immunomodulation would address both of these issues. The goals of this project are: (1) to optimize immunosuppressive therapy by defining a clinically applicable regimen for inducing transplant tolerance in heart transplant recipient, and, (2) to extend this approach to xenotransplantation, initially in a concordant primate model. Our preliminary data suggest the feasibility of applying the mixed chimerism approach to this problem, an approach already used successfully to allografts and xenografts in both rodents and cynomolgus monkeys. Specifically, we will: 1) establish the efficacy of the mixed chimerism approach as a means of inducing tolerance to heart allografts; 2) clarify the mechanisms leading to mixed chimerism-induced allograft tolerance; 3) establish the safety and efficacy of this approach as a means of inducing transplant tolerance in a primate concordant xenograft combination; and (4) test the effect of thymus transplantation on the induction of xenograft tolerance. An important objective is to clarify the role of direct versus indirect allo and xeno responses, in conventionally treated versus recipients undergoing mixed chimerism induction of tolerance. Each of the aims is based on preliminary data from the laboratories of the Principal Investigator and the other Project Leaders. These studies should provide valuable information both for determining the mechanisms involved in the induction of transplant tolerance and for developing a consistently effective therapeutic regimen that provides donor-specific non-responsiveness without the need for chronically administered immunosuppression. As such, these studies should have both theoretical and practical implications for current allograft recipients and for the eventual clinical applications of cardiac xenotransplantation.